Kinda... You have a MX 4000, that has 128MB of ram, 64bit DDR. Your friend has a FX550 same as mine and it has 265MB of ram, and 128bit DDR. So YES your friends is more advanced.
DDR stands for Double Data Rate-Synchronous DRAM. That is the technical name, and the technical expiation is that it can transfer data on both edges of the clock cycle. But check with website for a really through under standing: http://forum.pcmech.com/showthread.php?t=67892